This invention relates in general to chemical indicating devices and in particular to a new and useful device for determining the presence of sulfur dioxide.
Sulfur dioxide is a noxious substance which appears increasingly with the expansion of technology. The increasingly required monitoring demands simple to use and reliable measuring means.
A known indicator for determining sulfur dioxide in air uses the specific color reaction of moist zinc nitroprussiate or nitroprusside on a carrier of filter paper. The air is sucked through this indicator, and the resulting discoloration is evaluated with a color standard. The indicator is known in two types with different use. In the first type, the filter paper is impregnated with a solution of zinc nitroprussiate in ammonia solution and dried. Immediately before use, this indicator must be moistened by spraying with water so that it cannot react immediately. In the second type, the impregnation of the filter paper is effected with a solution of zinc nitroprussiate and solid ammonium acetate and adding glycerin. The hygroscopic action of glycerin makes moistening of the indicator before use unnecessary. The addition of ammonium acetate also ensures a good sensitivity. The stability of this indicator is very limited, however, since the reagent decomposes after a storage time of about 4 weeks (Analyst. vol. 85 (1960), S.147-148). A known indicator for determining sulfur dioxide in the atmosphere contains on paper or polymer film pararosaniline with a hygroscopic substance in gelatin or polyvinyl alcohol. In use, the indicator is exposed for a certain time to the environment to be monitored and the discoloration is evaluated according to a comparison method. Unsatisfactory is the crosssensitivity to nitrous gases, which is frequently found in emission controls together with sulfur dioxide (British Pat. No. 1,498,192).